MOW FLIES HANG ON. 71 



glass a hair 16 centimetres long, with a volume of liquid not exceed- 

 ing its diameter. Replacing the oil of olives with water, I obtained 

 the same result. The hair was 0*06 of a millimetre in diameter, and 

 the weight suspended may be calculated to have been 0*00045 of a 

 gramme. Repeating the experiment with horse-hairs, I found that a 

 hair 7*5 centimetres in length remained suspended under the same 

 conditions. The hair was 0'12 of a millimetre thick, consequently the 

 weight adhering to the glass was 0*00085 of a gramme. A hog's bristle 

 0*18 of a millimetre in diameter was suspended, although, being 55 

 millimetres long, it represented a weight of 0*00132 of a gramme. 



I also experimented with a hair ending in a bulb, which I formed 

 by holding the hair to a flame. I fixed to the glass a hair 0*06 of a 

 millimetre in diameter, terminating in a bulb 0*12 of a millimetre in 

 diameter, and weighing 0*00085 of a gramme, or the same as the horse- 

 hair previously used. 



The results of these experiments added weight to my supposition 

 that the liquid does not have to be viscous to enable the flies to stick. 

 To gain an absolute conviction, I weighed a number of flies, and found 

 their mean weight to be 0*045 of a gramme. I then ascertained the 

 number of hairs on the lower part of the pulvilli, and the size of the 

 extremities which they brought to bear upon the glass. It is not an 

 exaggeration to put the number on each pelote at 800 or 1,000 ; this 

 would give the fly a total of 10,000 or 12,000 hairs, by means of which, 

 with the assistance of a minute drop of liquid, it could support itself 

 on a solid body. It is proper to add, however, that a fly running on 

 a window has only three or four of its feet on the glass at a time, and 

 that therefore only half of its hairs, or 5,000 or 6,000 of them, are 

 serving it at once. I repeated my experiments, to determine the 

 weight hairs are capable of supporting when suspended in the man- 

 ner I have described, and found again that a hair 0*06 of a millimetre 

 in diameter will bear a weight of 0*00045 of a gramme ; of 0*12 milli- 

 metre, 0*00085 of a gramme ; and of 0*18 millimetre, 0*00132 of a 

 gramme, when the air is in motion. Then, according to my calcula- 

 tions, a fly would be able to walk upon glass, even if it weighed 

 0*020 of a gramme more than it actually does. I tested this by past- 

 ing little papers on the wings of flies to increase their weight. They 

 still kept themselves on the glass ; but they walked upward with 

 some difficulty when their weight was doubled. 



I perceived in the course of my experiments that the flies, espe- 

 cially the weighted ones, ceased to adhere to the glass when it was 

 moistened with the breath. Blackwall had essayed to explain this fact 

 by assuming that the sticky substance by means of which he supposed 

 they adhered mingled with the water, and was so much diluted by it 

 as to cease to be effective. I found, by examination with the micro- 

 scope, that this was not the case ; no mixture or dilution took place, 

 but rather a repulsion of the oily liquid by the water, and that that, or 



